Whether it’s feeling the coarse texture of jeans, the knotty bark of a tree, or the smooth skin of a loved one, our sense of touch is a critical aspect of how we interact with the world around us.
Now scientists say they have shed new light on why human fingertips are so sensitive, revealing that the culprit is hiding in our fingerprints.
It was already known that small receptors sensitive to mechanical stimuli are found in fingertip ridges, and they interact with two types of neurons in the fingers. The result is a “receptive field”: an area of skin where touch activates a particular neuron.
However, it was previously unclear how small a structure could be detected by each neuron. “You would expect a single papillary ridge to play a role, but that has not been proven [before], ”Said Dr. Ewa Jarocka, co-author of the study from Umeå University in Sweden.
To investigate the problem, Jarocka and colleagues asked 12 participants to each sit in a dental chair, with their right arm at an angle and their fingernails glued to a plastic retainer.
Writing in the Journal of Neuroscience, the researchers report using a robotic drumming device to run a surface of raised dots across each participant’s fingertips, with each dot being 0.4mm in diameter and the dots 7mm were separated from each other. The team monitored the response of some of the neurons in the fingertips using electrodes inserted into a nerve in the participant’s arm.
Using these responses, along with the position of the dots, the researchers were able to map the receptive field for each neuron. The results confirmed that each receptive field covered an area covering multiple fingertip ridges, and that there were highly sensitive zones within each field. But it also revealed that these zones were sensitive to a single dot – an object roughly the size of a fingertip back. In fact, they found stronger neural responses within these zones mapped to the fingertip ridges.
“It’s enough to deflect one ledge to evoke a neural response,” Jarocka said, adding up the held results regardless of how fast the drum was rotating or the direction it was moving.
Chris Miall, professor emeritus of motor neuroscience at the University of Birmingham who was not involved in the study, said the study increased understanding of how the information the brain receives from countless neurons represented the touch object.
“What the authors show is that the fine details of the ‘receptive fields’ of individual nerve fibers that come into contact with the mechanoreceptors in the fingertips closely match the valleys and ridges of the fingertips. So our very high sensitivity in the fingertips is because there are many, many nerve fibers with very small receptive fields, ”he said.
Prof Nathan Lepora, an expert in tactile robotics at the University of Bristol, agreed. “This work is welcome because it shows for the first time that areas of the skin felt by tactile neurons appear to align with the ridges of the fingerprint, showing that these ridges are directly involved in our sense of touch,” said he.
Miall added that while fingerprints also allow for better grip, the study highlights their role in detecting fine details on a surface. “Wearing gloves – even thin surgical gloves – has a huge impact. So think twice for anyone forced by the Covid pandemic to wear clinical gloves all day long, ”he said.